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Insight Into Cyanobacterial Circadian Timing from Structural Details of the Kaib–Kaic Interaction

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Insight Into Cyanobacterial Circadian Timing from Structural Details of the Kaib–Kaic Interaction Insight into cyanobacterial circadian timing from structural details of the KaiB–KaiC interaction Joost Snijdera,b, Rebecca J. Burnleya,b,1, Anika Wiegardc, Adrien S. J. Melquiondd, Alexandre M. J. J. Bonvind, Ilka M. Axmannc, and Albert J. R. Hecka,b,2 aBiomolecular Mass Spectrometry and Proteomics Group, Bijvoet Center for Biomolecular Research and Utrecht Institute for Pharmaceutical Sciences, Utrecht University, Padualaan 8, 3584 CH, Utrecht, The Netherlands; bNetherlands Proteomics Centre, Padualaan 8, 3584 CH, Utrecht, The Netherlands; cInstitute for Theoretical Biology, Charité-Universitätsmedizin Berlin, D-10115 Berlin, Germany; and dComputational Structural Biology Group, Bijvoet Center for Biomolecular Research, Faculty of Science, Utrecht University, Padualaan 8, 3584 CH, Utrecht, The Netherlands Edited by Jay C. Dunlap, Geisel School of Medicine at Dartmouth, Hanover, NH, and approved December 20, 2013 (received for review July 29, 2013) Circadian timing in cyanobacteria is determined by the Kai system We recently proposed a theoretical model for Kai that accu- consisting of KaiA, KaiB, and KaiC. Interactions between Kai rately described experimental phosphorylation dynamics, tem- proteins change the phosphorylation status of KaiC, defining the perature-step entrainment, and assembly dynamics of Kai pro- phase of circadian timing. The KaiC–KaiB interaction is crucial for tein complexes (22, 23). In this model, the formation of a the circadian rhythm to enter the dephosphorylation phase but it phosphorylated KaiC–KaiB complex counteracts the stimulation is not well understood. Using mass spectrometry to characterize of KaiC autophosphorylation by KaiA. Phosphorylated KaiC– Kai complexes, we found that KaiB forms monomers, dimers, and KaiB complexes sequester KaiA in KaiCBA complexes, thus tetramers. The monomer is the unit that interacts with KaiC, with preventing formation of “productive” KaiC–KaiA complexes. six KaiB monomers binding to one KaiC hexamer. Hydrogen–deu- This allows KaiC to switch back to its default autodephosphor- terium exchange MS reveals structural changes in KaiC upon bind- ylation activity. KaiA sequestration was demonstrated by semi- ing of KaiB in both the CI and CII domains, showing allosteric quantitative monitoring of Kai protein assembly with native MS. coupling upon KaiB binding. Based on this information we pro- Using this approach, we could confirm the formation of ternary pose a model of the KaiB–KaiC complex and hypothesize that Kai complexes bearing a high number of KaiA dimers, con- the allosteric changes observed upon complex formation relate comitant with the peak in KaiC phosphorylation. The formation to coupling KaiC ATPase activity with KaiB binding and to seques- of phosphorylated KaiCB complexes is thus a crucial step in tration of KaiA dimers into KaiCBA complexes. producing stable oscillations of phosphorylation, but the mech- anism by which these complexes trigger sequestration of KaiA is protein–protein docking | ion mobility spectrometry | native MS still unknown (17, 22–24). The structures of KaiC and KaiB have both been described yanobacteria represent some of the simplest organisms individually to atomic-level resolution, but structural details of Cknown to have an autonomous circadian timing mechanism. their interaction remain ambiguous. KaiC forms a hexamer with The core circadian clock of cyanobacteria does not rely on a double ring structure, the CI and CII domain each forming one transcription–translation feedback loops (TTFLs) as it does in ring, stacked together (4, 25, 26). Free KaiB has been crystallized higher organisms (1–4). This clock is known as the Kai system as a tetramer in four studies and has once been described as and it produces oscillations with an approximate 24-h period that a dimer (27–32). It was proposed to bind to KaiC as a tetramer are based on phosphorylation and assembly dynamics of the (33), dimer (27), or, more recently, as a monomer (32). From three Kai proteins: KaiA, KaiB, and KaiC (5). Phosphorylation single-particle EM reconstructions of KaiCB complexes, KaiB and assembly of Kai proteins determine their interactions with was suggested to bind to the CII domain of KaiC, which was circadian output kinases, which translate circadian timing in- supported by biochemical experiments where KaiCB interactions formation into 24-h rhythms of global gene expression (6–8). The Kai proteins can go through the same 24-h rhythm of phos- Significance phorylation and assembly when the system is reconstituted in vitro, simply by incubating KaiA, KaiB, and KaiC in the presence The Kai system is a widely studied model in theoretical biology of ATP (9). The in vitro oscillator is robust and can maintain and systems biology. It is to date the only known circadian a stable rhythm for weeks on end. It is temperature-compensated clock that can be reconstituted in vitro. Essential to the rhyth- and subject to temperature-step entrainment (9–12). It bears all micity of the system is the formation of the KaiC–KaiB complex. of the hallmark features of a true circadian clock and hence Many aspects of this interaction, such as the mode of binding of provides a unique opportunity to study biological timing to a high KaiB, the stoichiometry of the interaction, and the exact binding level of detail. interfaces have long remained ambiguous. We present a mass There are a large number of studies that describe the phos- spectrometry-based structural model of the KaiC–KaiB inter- phorylation dynamics and interactions between Kai proteins. action that answers many of these outstanding questions on KaiC consists of two domains, CI and CII, which possess ATPase the basis of direct experimental evidence. This structural model and phosphorylation activity, respectively. KaiC forms hexamers sheds light on the intricate workings of the in vitro oscillator. and is the central hub of the system (13–15). The default activity Author contributions: A.M.J.J.B., I.M.A., and A.J.R.H. designed research; J.S., A.W., and of the CII domain is autodephosphorylation. Unphosphorylated A.S.J.M. performed research; A.W. contributed new reagents/analytic tools; J.S., R.J.B., A.W., KaiC binds KaiA dimers, which triggers sequential autophos- and A.S.J.M. analyzed data; and J.S., R.J.B., A.W., A.S.J.M., A.M.J.J.B., I.M.A., and A.J.R.H. phorylation of KaiC at residues T432 and S431 (12, 16–19). This wrote the paper. “hyperphosphorylated” KaiC binds KaiB, which results in a The authors declare no conflict of interest. switch back to autodephosphorylation, first of T432, then S431. This article is a PNAS Direct Submission. Dephosphorylation proceeds through ATP/ADP phosphotran- 1Present address: UCB Celltech, Slough SL1 4EN, United Kingdom. ferase activity of KaiC (20, 21). The phosphorylation status of 2To whom correspondence should be addressed. E-mail: [email protected]. KaiC and the assembly with KaiA and KaiB define the phase of This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10. BIOCHEMISTRY circadian timing. 1073/pnas.1314326111/-/DCSupplemental. www.pnas.org/cgi/doi/10.1073/pnas.1314326111 PNAS | January 28, 2014 | vol. 111 | no. 4 | 1379–1384 Downloaded by guest on October 2, 2021 were assessed on the isolated CI and CII domains and by neg- ative-stain EM analysis of nanogold-labeled KaiCB (27, 32, 34, 35). However, it was recently reported that KaiB does not in- teract with the isolated CII domain of KaiC, but interacts with an engineered monomeric form of the CI domain instead (36). It was suggested in this study that CI–CII stacking interactions wedge the CI domains apart, exposing the putative KaiB binding site. The EM reconstruction of KaiCB complexes is ambiguous with respect to the stoichiometry of the interaction and the exact binding interface. Several small-angle X-ray scattering (SAXS) reconstructions of KaiCB suffer the same ambiguity in describing structural details of the interaction (33, 34). KaiB has been proposed to bind to KaiC with a final stoichiometry of either four or six copies of KaiB to six of KaiC (27, 32, 33). Moreover, there is currently no direct experimental evidence that pinpoints the KaiC-interacting region of KaiB. Here, we describe the MS-based structural characterization of Fig. 1. KaiB forms monomers, dimers, and tetramers. (A) Native MS of KaiB the KaiC–KaiB interaction, together with protein–protein docking at high (22 μM) and low (0.7 μM) concentration. A mix of monomers, dimers, predictions using HADDOCK (37, 38). We show that KaiB exists and tetramers is formed. The tetramer almost completely disappears at as monomers, in addition to tetramers. We demonstrate that KaiB lower concentrations. (B) Titration experiment of KaiB stoichiometry with binds KaiC with a final stoichiometry of 6:6 and that KaiB mon- native MS. The fraction of the total MS signal is used to calculate the ef- omers are the basic interacting unit. Using hydrogen–deuterium fective concentration of monomers, dimers, and tetramers. The averages of two experiments are plotted; error bars represent SDs. The effective KaiB exchange (HDX) MS, we have determined the structural changes monomer concentration is much more stable as a function of total KaiB in both KaiC and KaiB upon complex formation that revealed concentration, compared with the hypothetical situation where no tet- allosteric coupling between the CI and CII domains of KaiC upon ramers can form, which would correspond to a linear curve that is 1:1 for KaiB binding. Using this information to drive modeling with effective KaiB concentration as a function of total KaiB concentration. HADDOCK, we tested binding of KaiB to CI or CII against experimental ion mobility mass spectrometry constraints (39). Allosteric transitions are observed in both KaiC and KaiB that KaiB has been shown to bind preferentially to phosphorylated provide insight into the mechanism of KaiA sequestration and KaiC (18, 19, 42, 43).
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